Some designs of wireless communication devices—such as smart phones, tablet computers, and laptop computers—contain one or more Subscriber Identity Module (“SIM”) cards that provide users with access to multiple separate mobile telephony networks. Examples of mobile telephony networks include Third Generation (3G), Fourth Generation (4G), Long Term Evolution (LTE), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), Wideband CDMA (WCDMA), Time Division Synchronous CDMA (TD-SCDMA), Global System for Mobile Communications (GSM), Universal Mobile Telecommunications Systems (UMTS), evolved High Speed Packet Access (HSPA+), Dual-Cell High Speed Packet Access (DC-HSPA), Evolution Data-Optimized (EV-DO), Enhanced Data rates for GSM Evolution (EDGE), and single carrier Radio Transmission Technologies (1×RTT). A wireless communication device that includes one or more SIMs and connects to two or more separate mobile telephony networks using one or more shared radio frequency (“RF”) resources/radios is termed a multi-SIM communication device. One example is a dual-SIM-dual-standby (“DSDS”) communication device, which includes two SIM cards/subscriptions that are each associated with a separate radio access technology (“RAT”), and the separate RATs share one RF resource chain to communicate with two separate mobile telephony networks on behalf of their respective subscriptions. When one RAT is using the RF resource, the other RAT is in stand-by mode and is not able to communicate using the RF resource.
One consequence of having a plurality of RATs that maintain network connections simultaneously is that the RATs may sometimes interfere with each other's communications. For example, two RATs on a DSDS communication device utilize a shared RF resource to communicate with their respective mobile telephony networks, and only one RAT may use the RF resource to communicate with its mobile network at a time. Even when a RAT is in an “idle-standby” mode, meaning that the RAT is not actively communicating with the network, the RAT may still need to periodically receive access to the shared RF resource in order to perform various network operations. For example, an idle RAT may need the shared RF resource at regular intervals to perform idle-mode operations to receive network paging messages in order to remain connected to the network, etc. on behalf of the RAT's subscription.
In conventional multi-SIM communication devices, the RAT actively using an RF resource that is shared with an idle RAT may occasionally interrupt the active RAT's RF operations so that the idle RAT may use the shared RF resource to perform the idle RAT's idle-standby mode operations (e.g., paging monitoring and decoding, cell reselection, system information monitoring, etc.). This process of switching access of the shared RF resource from the active RAT to the idle RAT is sometimes referred to as a “tune-away,” as the RF resource tunes away from the active RAT's frequency band or channel and tune to the idle RAT's frequency bands or channels. After the idle RAT has finished its network communications, access to the RF resource may switch from the idle RAT to the active RAT via a “tune-back” operation.
Certain advanced RATs may have additional features. For example, an LTE mobile telephony network may be able to support more than one communications channel or transmit/receive chains using only one RF resource through carrier aggregation. A multi-SIM communication device may have an RF resource that supports a primary component carrier (PCC) and one or more secondary component carriers (SCC). The PCC may include an uplink carrier channel and a downlink carrier channel on a primary cell, and each SCC may be a downlink carrier channel on secondary cells. For example, a SIM with LTE CAT6 capability may include one PCC (uplink and downlink) and two SCCs, both used for downlink communications, i.e. receive chains. During a tune-away in such a device, the RF resource may shift the secondary cells used by the SCCs of the LTE subscription to the other subscription, which may be a GSM subscription or other 3G technologies. Thus, the SCCs may not be able to receive data from the network during the tune-away.
Another advanced feature of mobile telephony networks is antenna diversity. Wireless communication devices may include multiple wireless antennas configured to receive data on a single wireless link. Antenna diversity may enhance reliability by minimizing the channel fluctuations due to fading. For example, multiple-input multiple-output (MIMO) operation may be used to receive wireless signals through multiple antennas at the same time corresponding to multiple transmitting antennas from the base station. MIMO communications takes advantage of receiving signals along multiple, different paths (multipath) that adds a spatial dimension to signal reception, which can be used in processing the received signals to increase performance. For example, in an LTE subscription with two SCCs, the SCCs may be configured for MIMO communications such that each SCC may support a separate receive chain through each antenna. In the absence of MIMO, the two SCCs may be only capable of supporting a single downlink channel because the antennas are correlated or interfere with each other.
Wireless communication devices may also include multiple wireless receive chains configured to receive data on more than one wireless link. For example, in some wireless communication systems or markets, a wireless service provider may implement more than one type of RAT or air interface protocol within a single system. Wireless communication devices that are configured with multiple receive paths may be capable of using one or more receive paths to communicate on more than one RAT at a time. Such devices, which may be referred to as a hybrid device, can therefore use a diversity antenna/receive chain to tune away to a network implemented by different carriers (e.g., using multiple SIMs), and/or the same carrier (e.g., in a hybrid system). In other wireless communication systems, wireless communication devices may be configured with multiple SIMs, each of which may be configured to communicate with different networks. Therefore, receive chain configurations may provide wireless communication devices with a variety of tune-away options, such as tuning away to a network associated with the same carrier, associated with a different carrier in the same radio access technology, associated with a different carrier in a different radio access technology, etc.
Wireless communication devices send, or report, quality metrics to the network for each communications channel. These quality metrics include a channel quality indicator (CQI) and a rank indicator (RI). These quality metrics may affect the resources that the network allocates to the channel. During tune-aways, the wireless communication device may lose certain high-speed data capabilities on certain channels and, as a consequence, report lower quality metrics for the affected channels. In response, the network may allocate lower system resources to the affected channels, such as reducing the size of resource blocks allocated to the channel or changing the modulation and coding scheme for the channel to increase redundancy. When the tune-away ends, the wireless communication device may report improved quality metrics for the affected channels, but the network may take a long time to allocate more resources to the channel. In the meantime, the lowered resources for the channel may persist and thus the channel may not operate as efficiently.